Abstract
The UK water industry is currently going through an optimisation phase of anaerobic digestion assets, upgrading sites to ‘super centres’ with Advanced Digestion technologies increasing digester loading and dry solids feed whilst at the same time reducing retention times. As technological advances push digesters ever harder, effective mixing becomes more and more critical to successful operation.
This paper looks at the mixing fundamentals and the use of sludge rheology to design effective systems for thick sludge digestion along with the effects of thermal hydrolysis pre-treatment. This paper reports on the performance of the sequential gas mixing at Aberdeen after 7 years of operation. It also presents the recent digester mixing installations in the UK latest Advanced Digestion sites, namely Riverside STW and Davyhulme WwTW where pump and sequential gas mixing systems have been respectively installed.
Finally the paper reviews what lessons from digester mixing in the water industry can be exported to the burgeoning Biowaste industry and asks if future trends such as co-digestion will have an effect on mixing technology selection.
Key words
Digester Mixing, Jet, Sequential Gas, Advanced Digestion, Co-Digestion
Introduction
The UK water industry has embraced an optimisation cycle of its digestion assets by upgrading sites to ‘super centres’ with Advanced Digestion technologies. These processes enable the digester loading and dry solids feed to be increased while at the same time reducing the digester retention times. As operational experiences and technological advances push digesters ever harder, effective mixing becomes more and more critical to successful operation.
Digester mixing has always been recognised as a key process unit and whilst a variety of mixing technologies have been recently implemented on AD sites, the primary objectives of a digester mixing system remain the same and are namely:
- Provide contact between the feed sludge and the active biomass to maximise gas production.
- Provide physical, chemical and biological uniformity within a digester in order to maintain a satisfactory environment for both acid and methane forming bacteria
- Distribute organics and dilute inhibitory substances
- Prevent stratification and temperature gradients
- Utilise the digester volume effectively to maintain the required biomass residence time, i.e. minimise short-circuiting
- Minimise the deposition of solids, as this reduces effective digester volume.
- Minimise foaming and the formation of a surface scum layer.
The first three mixing objectives are related to blending the feed with the digesting sludge. The rate of blending is characterised by a time (the blend time) taken to reach a specified degree of uniformity. The biomass residence time is influenced by the flow pattern and residence time distribution (RTD) in the digester. Significant stagnant or dead regions reduce the effective or ‘active’ volume and hence reduce residence time. The active volume in the digester is characterised as a percentage of the total sludge volume.
This paper revisits one of the first digester mixing system specifically designed for thick sludge digestion and gives an operational performance update. It also presents the recent digester mixing installations on the UK latest Advanced Digestion sites, namely Riverside STW and Davyhulme WwTW where pump and sequential gas mixing systems have been respectively installed.
Finally the paper reviews what lessons from digester mixing in the water industry can be exported to the burgeoning Biowaste industry and asks if future trends such as co-digestion will have an effect on mixing technology selection.
